3Com 3C17300-US - SuperStack 3 Switch 4226T, Implementation Manual

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56 C
HAPTER
6: U
SING
T
RAFFIC
P
RIORITIZATION
3 Create Profiles The next step is to create a profile, which associates
classifiers with service levels.
4 Apply QoS profile After a QoS profile has been created, it can be
assigned to the Port(s). When the profile is assigned to the port(s), the
QoS configuration defined in the profile will immediately become active.
Head of Line Blocking (HOL)
You can adjust the Head of Line Blocking settings for Fast Ethernet ports
to one of the following:
■
QoS — This is the default setting. Using this setting, the Switch uses a
maximum of 12 packets of egress buffering per port (potentially 18
kilobytes) for normal priority traffic. This gives the flexibility to ensure
that high priority traffic can be provided with sufficient extra
buffering. Ingress flow control is not normally required with this
setting. Data protocols operating window sizes greater that 18
kilobytes will not work efficiently with this setting.
■
Data — This setting increases the maximum egress buffer per port to
32 kilobytes or 27 packets. This setting is suitable if data applications
require a window size of up to 32 kilobytes. Under these
circumstances, high priority traffic may not always be able to access
sufficient Switch buffers to guarantee the expected QoS.
■
Advanced — You should only use this setting if you are an
experienced network administrator. It allows you to set HOL to suit
certain specialist applications. It has two modes:
■
Disable — This mode disables HOL completely. All egress and
ingress buffers will be used on demand and QoS settings will be
largely ineffective in buffer-overload situations. When ingress
buffers are exhausted, flow control will operate.
■
Enable HOL with specific HOL values — This allows you to set a
value between 10 and 50 kilobytes of egress buffering.
dua1730-0bAA03.book Page 56 Monday, July 11, 2005 11:14 AM